In the past, many have proposed using nanoparticles as contrast agents to illuminate a reservoir or a hydraulic fracture. Some have proposed using nanoparticles to increase the conductivity of a reservoir. Others have proposed using magnetic nanoparticles to enhance magnetic permeability of the reservoir and change the velocity of electromagnetic waves propagated in the reservoir. Additionally, some have proposed using nanoparticles to change the electrical permittivity of the reservoir.
All of the proposed methods discussed above change only the phase or amplitude of the electromagnetic waves. None of these methods change the frequency of the electromagnetic waves. This is a major factor that limits the effectiveness of these methods. This is because the reflected electromagnetic waves from the rest of the formation overlap with the electromagnetic waves reflected from parts of the reservoir filled with nanoparticles in both time (due to limited bandwidth) and frequency. This significantly limits the sensitivity of the receiver, due to the interference caused by strong echoes reflected from the rest of the formation, boundaries of the horizontal-well, metallic objects (equipment) used in hydraulic fracturing, and/or the direct-coupling between the transmitter and receiver in the main transceiver. The problem is illustrated in the example shown in FIGS. 1a-1b. As shown in FIG. 1b, a transmitter may send a signal to proppants located in a formation, and the reflected signal may be received by a receiver. However, the receiver may also receive echo signals reflected from the formation and a direct coupling signal from the transmitter. As shown in FIG. 1a, the overlap between electromagnetic waves from direct coupling and reflected from the formation result in significant interference to the reflected signal, thereby making it difficult to accurately receive and detect the small fractures waves. As a result, it is difficult to map the proppants for fracture mapping utilizing the above noted methods.
This problem is similar to the problem of clutter in radar. Clutter is essentially the strong echoes reflected from undesired stationary objects (e.g. ground or background material) that overlap with signals reflected from a desired small, stationary object. This issue is resolved in Doppler radar, because the signals reflected from a moving object differ in frequency from signals reflected from stationary objects. Due to the frequency-change, these two signals can be separated in the frequency domain.